The security documents market strives to offer tamper-resistant ID cards and secure documents which cannot be forged or counterfeited and are of a multi-layered structure. In this market, there is a need to have an overlaminating film that has excellent adhesion to a variety of substrates including but not limited to photographs or heavy ink coverage on paper or plastic materials. In addition to a strong adhesive bond, the film needs to be tamper-resistant. A tamper-resistant film is defined herein as one which cannot be separated from the laminated document without destroying the multi-layered construction, rendering it open to data alteration. Once altered, a document having a film that is not tamper resistant would be able to be re-laminated by skilled counterfeiters using the same overlaminate structure complete with any additional security features.
One important secure document test is a hot plate intrusion test. This procedure attempts to simulate a counterfeiter who heats a laminated structure, pries the layers apart, alters the data, and then re-laminates the structure. During this procedure, a laminated piece (film laminated on both sides to TESLIN® synthetic printing sheet) is placed on a tightly temperature-controlled hot plate. The hot plate used for intrusion testing is sold by Torey Pines Scientific Model HP30A. TESLIN® sheet, available from PPG Industries, Inc., is a polyolefin-based printable porous and waterproof bright white synthetic printing sheet. This is a single-layer uncoated film.
In proceeding with the hot plate intrusion test, the laminated film pieces are heated to a point where the thermal adhesive softens and flows while the substrate remains rigid. A blade is used to help separate the layers. Most thermal laminating films will easily delaminate because the substrate is an oriented polyester terephthalate (OPET) with a high crystallinity. The adhesive becomes soft well before the substrate does, allowing for the separation of the two layers. In such a situation, there is a potential of separating the film without damaging the substrate, thereby facilitating counterfeiting.
We have determined that a property relevant to separation is the glass transition temperature of the substrate (namely, the glass transition temperature where the substrate begins to exhibit rubbery, flexible tendencies). We have further come to realize that polymers with too much crystallinity may have a glass transition point (the amorphous portions of the structure going through this change), but the polymer substrate may still remain rigid due to the remaining crystalline structure. In contrast, an amorphous polymer or a polymer with low crystallinity becomes pliable once the glass transition of the substrate is exceeded, and a clean separation which relies upon a rigid backing now fails. A crystalline or semi-crystalline polymer (common in film applications) will have a difficult time passing this hot plate intrusion test due to the rigidity remaining in the structure until the melting point, typically above about 300° F. (or above about 146° C.), is reached. A base substrate produced from an amorphous polymer (or polymer with a low crystallinity) will provide a more secure product.
An additional test that security laminations must pass was developed by Polaroid Identification Systems, currently Digimarc Identification Systems. In this test, often called the Polaroid environmental test, films are laminated to Teslin® synthetic printing sheet. The films are cut into CR80 sized (3.37 inch by 2.13 inch or about 8.6 cm by 5.4 cm) pieces or cards and placed into a pressure cooker with 225 mL of distilled water. The cards are not submerged in the water but elevated above it. The pressure cooker, with the cards inside, is placed into an oven at 160° F. (71° C.) and cooked for five days. At the end of the five days, exactly 15 cards are placed into a rust free paint can with 30 mL of distilled water and 20 grams of 120 grit sand. The paint can is sealed and shaken for three hours. The paint shaker used for the Polaroid environmental test is a Miller paint shaker Model G70. The pieces are removed and looked at for delamination. “Dog-eared” corners and delamination are unacceptable and considered failures. Rounded corners due to abrasion without delamination are acceptable. Other variants of this test may exist and be employed to simulate similar “severe” weathering conditions.
One other test of importance is the peel test measured using the following method. Two pieces of the coated film are cut to a size of 4.5 inches by 5.5 inches (11.5 cm by 14 cm) with the cut of 5.5 inches being parallel to the machine direction of the film. These two pieces then are sandwiched together with the adhesive inside the polymer film on the outside surfaces. The structure looks like the following from top to bottom: polymer film/adhesive//adhesive/polymer film. A piece of paper 1.25 inches by 5 inches (3.2 cm by 12.7 cm) is cut to be used as a separation tab. This is inserted along one end of the peel test sandwich parallel to the cut of 4.5 inches. The entire sandwich is placed within a silicone coated paper carrier and laminated through a standard pouch laminator. The laminator produces a temperature at the bond-line, the adhesive//adhesive interface, of 240° to 250° F. (115.5 to 121° C.), which is the ideal temperature at this stage of the peel test. For a product that is 0.020 inches (about 0.5 mm) in thickness (7/3//3/7) the laminator is set at 300° F. (146° C.). After lamination, test strips 1 inch by 5 inches are cut from the sample perpendicular to the paper insert.
These test strips are subjected to this peel test as follows. A Lloyd tensiometer is used to measure the force to separate the laminated pieces. After zeroing the test equipment, a strip is inserted into the jaws so that the jaws are gripping the paper tabs. Depressing the “GO” button will start separation of the jaws at a rate of 10 inches per minute. A load cell attached to the top jaw measures the force applied between the jaws, which is the force to destruct the sample. The average pull force is noted along with the mode of failure: adhesive (separation internal to the laminating film at the interface between the polymer film and the adhesive), cohesive (separation between the adhesive//adhesive interface), or tear (tear of the polymer film).
According to a variation of this peel test, a piece of 0.010 inch (about 0.25 mm) Teslin® polyolefin-based synthetic printing sheet (available from PPG) can be inserted between the coated pieces, which is usually how the product is used in the field. The test is conducted in the same manner, with the following exceptions: 1) the laminator is typically set at 350° F. (121° C.) to allow for the additional 10 mil (about 0.25 mm) of thickness, and 2) the mode of failure could (and should) be Teslin® failure in the “Z” direction.
Even though the majority of secure documents are flush cut, i.e. there is no adhesive//adhesive bonding, this peel test demonstrates value in determining if the polymer film/adhesive interface is the weak link. A cohesive failure (adhesive//adhesive failure) is a preferred benchmark. This mode of failure requires destructive physical force at the adhesive//adhesive interface, demonstrating that a strong bond between the polymer film and adhesive exists.
A current approach in the security laminate market is supplied by Transilwrap and consists of an amorphous polycyclohexylenedimethylene terephthalate (PCTA, Eastman A-150 extruded at a thickness of 0.007 inch or about 0.18 mm by Pacur, LLC) substrate coated with a low density polyethylene (LDPE, AT Polymer 418, extruded at a thickness of 0.003 inch or about 0.076 mm) thermoplastic adhesive. This film may have other security features printed on the adhesive, such as UV-only visible security printing, color shifting security printing, Advantage™ security label, or other printed optical variable devices (OVD).
Concerning this current Transilwrap product, LDPE is not the most aggressive adhesive agent. Being a straight chain hydrocarbon, there is not much functionality that allows adequate adhesion to printed surfaces. Thus, corona treatment is used to add functionality (primarily carbon-oxygen single and double bonds) to the surface and increase the bonding potential of the adhesive. As print technology has advanced, simple corona treatment does not provide enough functionality for acceptable adhesion. Additionally, the saturated single bond chemistry often proves inadequate for suitable adhesion to the rigid substrate and fails with an adhesive failure mode.
Changes to LDPE technology have caused declining results in the Polaroid environmental test when using LDPE in this manner. These changes include a reduction in the level of unsaturation (carbon-carbon double bonds) in the polymer chains by manufacturers of LDPE such as Dupont, Equistar, and AT Polymers. Fine-tuning the ethylene resin to be more saturated for most end users is beneficial due to lowering instances of gels thus yielding a cleaner material. However, this in turn weakens its ability to be a strong adhesive. We have determined that this change has impacted the current Transilwrap LDPE coated product in two negative ways. First, by decreasing the level of unsaturation, the number of active bonding sites also is decreased since the unsaturated bonds are more readily oxidized by traditional corona treatment. Second, decreased adhesion to the PCTA substrate has been realized.
In typical manufacturing approaches, an optional primer layer is applied to the polyester film prior to extrusion coating. When primers are utilized, they can take the form of thin-layer, water-resistant primer material where the cards being assembled are to withstand exposure to moisture during use, for example. In general, the primer bonds best to unsaturated resins; whereas, the recent raw material trend has been to become more saturated, i.e. less unsaturated. For example, a slightly increased adhesion of the LDPE to the PCTA substrate is observed. Due to the unfavorable economics and the need in other markets to reduce the level of unsaturation of commodity-grade LDPE, this solution is not suitable.
Another product manufactured by Transilwrap is a PCTA substrate coated with a blend of ethyl ethyl acrylate copolymer (EEA) adhesives. This product has excellent bond strength to a variety of print technologies and has not seen the intermittent failures through the Polaroid environmental test of other products. However, the EEA resins have a softening point that is 35° C. lower than glass transition point of the PCTA. As discussed herein, and in keeping with the present invention, it has been determined that such a large difference between those respective temperatures is a primary reason why this product does not pass the hot plate intrusion testing. An important objective of the present invention is to make the two respective temperatures (softening point of the adhesive and glass transition temperature of the substrate) virtually one and the same.